Three phase switching regulators that utilize DC to AC conversion require a three phase bridge circuit. Conventional bridge circuits consist of three pairs of complementary switches utilizing either transistors or SCRs as the switching element. Each complementary pair alternately switches a positive and negative DC voltage to one of three respective output terminals. Each of the three complementary pairs are controlled respectively by three square waves that are 60.degree. out of phase. The output terminals for each of the complementary pairs are connected such that the bridge generates a pseudo square wave. The operation of a three phase bridge circuit is described more fully in co-pending U.S. application Ser. No. 345,699.
When transistors are utilized as switching elements, practical limitations of the devices must be considered. The transistors in each complementary pair are driven by a square wave such that each switch is 180.degree. out of phase with its complement. Typically, a transistor can be turned on faster than it can be turned off. This is due to the storage time within the device itself. If the transistor is not allowed to completely turn off prior to turning on the remaining transistor in the pair, then a momentary short is impressed across the positive and negative DC terminals. This allows a high current to pass through both transistors in the pair which is normally termed "shoot-through" current. This shoot-through current increases the transistor failure rate in addition to causing perturbations in the output waveform.
To prevent shoot-through current, systems have been developed that inject a delay in the system to prevent a transistor from turning on before its complement turns off. This delay is adjusted such that the transistor that is turning off is allowed sufficient time for its conduction storage to be swept out. Typically, in the type of transistors utilized, three to five microseconds of storage time is present.
One type of delay circuit that has been implemented utilizes an accurate delay circuit. This type of circuit monitors each individual transistor to ensure that the transistor is off prior to turning on its complement. As soon as the transistor has turned off, the complement is turned on. The disadvantage to this type of circuit is that asymmetry will be present in the output waveforms. Depending on the characteristics of the transistors, it is possible to have one delay in one complementary pair of transistor switching elements and a second and different delay in a corresponding pair. When driving a motor with the three phase output waveform, the asymmetrical delays can result in DC current input to the motor and undesirable harmonics.
In view of the above problems, it is desirable to have a three phase bridge circuit with a uniform delay between the turning off and the turning on of two transistors in a complementary pair such that a symmetrical waveform is output.